1. Field of the Invention
The present invention relates to an electronic part used for electronic apparatuses of various types and a method for fabricating the same, and more particularly, to an electronic part fabricated by intaglio printing and a method of fabricating the same.
2. Description of the Related Art
In recent years, as electronic apparatuses become smaller, electronic parts used in the electronic apparatuses are more and more miniaturized. Under such conditions, conductor patterns are required to fulfill the following requirements: to have a fine conductor line (hereinafter simply referred to as line) constituting the conductor pattern; on increase in thickness of a conductive film constituting the conductor pattern in order to lower line-resistance; and to have a multilayered structure in order to miniaturize the electronic parts.
Conventional conductor patterns of the electronic parts are fabricated by printing a pattern of conductive paste such as a silver paste and a copper paste on an object (a substrate) employing printing techniques such as screen printing and intaglio printing, and then burning the printed object. As an application of the intaglio printing, there is the following printing method for forming a desired conductor pattern as disclosed in Japanese Laid-Open Patent Publication No. 4-240792. An intaglio plate corresponding to a conductor pattern to be formed is filled with conductor paste (organic metal ink). After the conductive paste is dried and cured, the pattern is transferred onto a substrate with a curable resin therebetween. As a result, a desired conductive pattern is formed.
In addition, for a hybrid IC circuit, a thermal head, or a transparent electrode, a method utilizing thin film formation and etching may be used. This is because the width of each line and the interval between the lines in the conductor pattern can be made finer by this method. The process of the method is as follows. A thin film made of a conductive material such as gold, aluminum, ITO or the like is formed by vapor deposition or sputtering. A mask pattern corresponding to the desired conductor pattern is formed by photolithography using a light-sensitive resin. Then, the thin film made of the conductive material is etched using an etchant and the mask pattern. Lastly, the light-sensitive resin is removed, thereby forming the conductor pattern.
The above conventional methods, however, have the following problems.
Conventional screen printing can be carried out with equipment at relatively low cost. Moreover the number of steps required to form the conductor pattern are few. However, it is very difficult to form a fine conductor pattern so that the width of the line of the conductor pattern to be formed is 70 .mu.m or less by screen printing. Furthermore, it is also difficult to reduce a line pitch to 150 .mu.m or less. In addition since the conductor pattern is uniformly formed by screen printing, it is impossible to make a difference in level (difference in height of the line) of the pattern according to design requirements.
It is possible to form such a fine conductor pattern so that the width of the line is about 50 .mu.m and the line pitch is about 100 .mu.m by conventional intaglio printing. However, it is difficult to form a conductor pattern having a thickness of 5 .mu.m or more. Therefore, any decrease in the conductor resistance is limited to a certain extent.
On the other hand, making a fine conductor pattern in each layer is sometimes insufficient for obtaining the electronic parts with the desired high-density. Therefore, it is required to form a multilayered structure. Such a multilayered structure is formed by laminating several sandwich structures respectively including a lower layer conductor pattern, an insulating layer and an upper layer conductor pattern. In this case, it is necessary to form via holes for connecting conductor patterns of upper and lower layers to each other, which also necessitates making the via holes fine according to the fine conductor patterns. However, in conventional printing methods including the aforementioned method in Japanese Laid-Open Patent Publication No. 4-240792, it is very difficult to form the via-holes having a diameter of 100 .mu.m or less.
Moreover, it is required to form electrodes for connecting the upper and lower layers within the via holes in order to obtain a reliable electrical connection between the conductor patterns of both layers. However, even if fine via holes having a diameter of 100 .mu.m or less can be formed, it is difficult to form the electrodes in the via hole of such a small size by conventional methods.
In conventional intaglio printing, an intaglio plate made of rigid material such as glass or silicon wafer As generally used. In such a case, during the step of transferring the conductor pattern onto the object such as a ceramic substrate or a glass substrate with the curable resin therebetween, the intaglio plate is not sufficiently flexible when the intaglio plate and the object attached to each other are to be separated. As a result, a strong peeling force is required to separate them from each other off their surfaces.
In order to solve the above problems, a metal sheet having flexibility may be used as an intaglio plate. However, even in such a case, the shaping process (formation of a groove) of the intaglio plate is performed by wet etching which is isotropic etching, Therefore, the shaping of an intaglio plate with a high aspect ratio, that is required to form such a conductor pattern that the conductor film is thick (in other words, the line in the pattern is high) with respect to the width of the line, cannot be carried out.
On the other hand, as can often be seen in semiconductor techniques, the formation of the conductor pattern utilizing photolithography is useful in forming the pattern of a small area with the width of line of several .mu.m or less. However, for forming the conductor pattern used in the electronic parts, it is generally required to form a pattern of a relatively large area. In such a case, it is required to conduct a series of steps including deposition of the conductive film, application of a resist, exposure, development, etching, and removal of the resist with large equipment. Since such equipment is generally expensive, the cost for fabricating the conductor pattern is likely to increase.